Use of Modulation Transfer Function in Determination of Focal Spot Dimensions

The modulation transferfunction (MTP)has been used in the imagingfield to evaluatefidetity of imaging system< in reproducing different spatial frequencies. It has been usedfor similar purpose in radiographic imaging also. As the focal spot size has an infiuence on the resolution of a system it should be expacted to hiflueuce MTFtoo. Sucb qualitative linkage between focal spot size and MTFhas been known. Thispaper demonstrates definitive relationship between the two which wads to a simpw technique to obtain focal spat dimension from tbe spatial frequency at which MTFbecomes zero or undergoes a minimum. This approach is applied to conventional and microfocns X-ray units. MTPcurves are drawn by three different methads viz. tbe hasic method, the PFTof line spreadfunction (LSP)andfrom the derivative oftbe edge spreadfunction. Results were compared with resolution (star) pattern method and were found in agreement. except with a bias towards overestimation in the case of microfocal spots when £SFmethad is used. The metbod gave lmver values as compared to pin how method for conventioad units.